The Leydig cell produces testosterone in mammals. Testosterone is required for the development of the male reproductive tract, for embryonic and neonatal "imprinting" of testosterone responsive structures such as the hypothalamus, prostate and liver, for potency and libido in the adult make, and for testicular, epididymal and accessory sex organ function in the adult. Yet, little is known regarding the factors regulating development of and sustaining steroidogenic function in Leydig cells. Therefore, we propose studies in the neonatal rat designed to: first, identify transitional cells intermediate between primitive mesenchymal precursor cells and the adult Leydig cell; second, describe unequivocal morphologic and biochemical markers for such intermediate transitional cells; third, study the regulation of this Leydig cell developmental process both in situ and in vitro. In the adult rat, we propose studies designed to: first, elucidate the "trophic" effect of LH on Leydig cell division, number and intratesticular mitogenic activity and transforming growth factor beta content; and second, test the effect of LH on Leydig cell peroxisome structure and function.